The skies are raining big chunks of ice, and experts ranging from scientists to federal investigators are scrambling to learn what's going on.

For the second time in a week, California was the victim of an aerial, icy assault, the latest being early Thursday when a chunk of ice the size of a microwave oven plunged out of a cloudless sky into the San Bernardino County town of Loma Linda. The ice punched through the metal roof of a recreation center, leaving a hole up to 2 1/2 feet wide, then fragmented into opaque, brilliant white chunks, one as big as a bowling ball. No one was hurt.

Two tennis players were batting a ball around outside the Drayson Center at Loma Linda University on Thursday morning when they heard a strange sound, said Rolland Crawford, Loma Linda Fire Department division chief.

"They described it as the sound an artillery shell would make -- shoosh, shoosh," he said. "They looked up. They didn't see the ice, nor did they see a plane."

A similar incident occurred last Saturday in Oakland, where a plunging ice ball plowed into a field at Bushrod Park on Shattuck Avenue and blasted out a crater up to 2 feet wide. Again, no one was hurt.

The simplest, least controversial hypothesis is that the ice was dropped from airplanes, but there's little direct support for that view. A few experts who study such phenomena have suggested that similar occurrences around the world owe more to exotic causes, perhaps even global warming.

The Federal Aviation Administration is investigating the two latest cases under the theory that the ice fell from an aircraft, FAA spokesman Mike Fergus said Friday.

Such cases can be very tough to solve, he said.

In both cases, the ice was clear or whitish -- not bluish, as one would expect of ice that had leaked from an airplane's restroom, for instance.

The agency has traced blue-ice cases back to aircraft, but Fergus said that is extremely difficult to do. One of the many reasons is that wind speed and direction can vary at different altitudes and often change suddenly. As a result, the otherwise smooth trajectory of falling ice can be radically altered.

For this and other technical reasons, investigators can't always be sure whether a particular piece of ice fell from a particular airplane even after they've reconstructed the probable plane's flight path, Fergus said.

Legends about plunging ice go back for centuries. They didn't begin to receive serious scientific attention until a few years ago, however, when Spain and other countries were pelted by the mystery intruders.

Possible explanations range from the mundane to the bizarre.

One theory is that ice is somehow forming on the outside of aircraft, perhaps in areas that aren't protected by deicing equipment, said David Travis, a climatologist at the University of Wisconsin at Whitewater. Last year, he and 11 others co-wrote an article on the ice-fall mystery in the Journal of Atmospheric Chemistry.

Lead author Jesus Martinez-Frias of the Planetary Geology Laboratory in Madrid and his colleagues have collected reports of 40 cases around the world since 1999 of puzzling falling ice, or "megacryometeors," as they call the strange objects.

Martinez-Frias hypothesizes that the ice forms in the upper atmosphere by a process similar to the formation of hail inside thunderstorms but without a thunderstorm.

Inside a thunderstorm, hail forms like this: Extremely violent vertical winds can repeatedly blow water vapor up and down, like clothes inside a washer. In this process, moisture rises to a high, cold altitude, where it freezes. Then its weight causes it to plunge back toward Earth -- until new winds blow it back to a high altitude, causing it to gain an additional layer of ice.

The process occurs over and over again until the object falls to Earth as a hailstone, sometimes as big as a baseball and occasionally bigger.

But how can ice fall from a cloudless sky? Martinez-Frias speculates that global warming is causing the lower part of the atmosphere -- the troposphere, where we live -- to expand and rise. This means that the tropopause, which is the so-called roof of the troposphere, is forced to a greater height, where it cools more than normal.

Thus, he suggests, the new, steeper temperature difference between warm and cold air in the upper atmosphere generates turbulent up-and-down winds that repeat the hail-formation process, without a thunderstorm.

Although he co-wrote the paper on megacryometeors with Martinez-Frias, Travis is leaning toward the aircraft theory for what happened in Oakland and Loma Linda. "I've talked to a lot of pilots who tell me there are places on airplanes where the deicing equipment doesn't cover," Travis said Friday.

Then there may be the conspiracy theorist who might suspect the ice is falling off the kinds of supersecret spy planes that the military tests in the Southern California and Nevada deserts. But spokesman John Haire of Edwards Air Force Base in the Mojave Desert said any such theorists are all wet: "We don't do test flights over Loma Linda or Oakland."